1. Field of the Invention
The invention relates to a white, biaxially oriented film made from a crystallizable thermoplastic with high whiteness, the thickness of the film being in the range from 10 to 500 μm. The film comprises at least one rutile-type titanium dioxide as pigment and an optical brightener, with or without other additives, and has at least one additional functionality. The expression “additional functionality” includes UV resistance, yellowing resistance, photooxidative stability, flame retardancy, and sealability. This film may have one or more of the functional properties. All of these films are capable of cost-effective thermoforming, have good orientability, and have very good optical and mechanical properties. The invention further relates to a process for its production.
2. Description of the Related Art
White films with a thickness from 10 to 500 μm are well known. The whiteness of these known films is very generally ≦80%. The films also have an undesirable yellow tinge, i.e. are not brilliant white. An example of the Yellowness Index, which is thickness-dependent, is >45 for films of from 10 to 100 μm thickness.
DE-B 23 46 787 describes a flame-retardant raw material. Besides the raw material, its use to give films and fibers is also claimed. The following shortcomings were apparent when film was produced using this claimed phospholane-modified raw material: the raw material is very susceptible to hydrolysis and requires very good predrying. If the raw material is dried using dryers of the prior art it coagulates, making it extremely difficult to produce the film. The films produced, under conditions which are extreme and not cost-effective, embrittle when exposed to heat, i.e. their mechanical properties deteriorate sharply due to rapid onset of embrittlement, making the film industrially unusable. This embrittlement arises after as little as 48 hours of exposure to heat.
None of the knowledge encompassed by the prior art teaches or implies the existence of a white, biaxially oriented thermoplastic film with a thickness from 10 to 500 μm which has at least one additional functionality, its properties being retained during processing and use.